The present invention relates to a method and a device for tracking eye movements.
Refractive laser surgery of the cornea, in particular with ablative excimer lasers (ArF—wavelength 193 nm) as OP laser, has become an established method of treating refractive sight defects. The treatment is carried out with so-called spot scanning systems on the patient's freely-moving eye. These systems also allow patient-specific corrections, so-called customized ablations.
A patient's eye movement makes it necessary to guide the beam of the OP laser in line with the movement. To this end the patient's eye movement and thus the current eye position is recorded with a so-called eye-tracking system and used as adjustment value to steer the OP laser. Customary eye-tracking systems work on the basis of image-processing systems in which these can for example measure the pupil of the eye using a digital camera and determine the eye position with image-processing algorithms by comparing image sequences. As eye movements take place very rapidly and as even the smallest angle errors have to be recognized, high-speed video cameras are normally used for this purpose.
The pupil is preferably used as tracking object for the high-speed video camera, as an automatic video-supported pupil centering can thereby be carried out at the start and the same object can also be tracked during the treatment. This procedure is problem-free on an untreated eye. However, already during preparatory operations such as the removal of the epithelium during PRK (photorefractive keratectomy) or lifting up the flap during LASIK (laser-assisted in-situ keratomileusis), the contrast ratios are changed. During the laser treatment this problem is further aggravated, as a strongly dispersive surface beyond the diameter of the pupil is created.
To avoid this problem different optimization approaches have been worked out. There is a switch to other tracking objects on the eye such as e.g. limbus or iris after initial pupil centering or additional tracking rings are placed on the eye in order to ensure a stable functioning of the eye tracking during the treatment. However, this entails unwelcome additional treatment costs.
In another optimization approach the eye is illuminated from the side with infrared LEDs. A lower failure rate of the system is thereby also achieved during the laser treatment.
However, the illumination necessitates additional work for the doctor and a limitation of working freedom in the OP field.